1. Field of the Invention
The present invention relates to a fault detection system applied to a control apparatus for an internal combustion engine of an automotive vehicle, or more in particular to a system for detecting a fault of a subcontrol apparatus for controlling an actuator in place of a main control apparatus of an internal combustion engine when the main control apparatus becomes out of order.
2. Description of the Related Art
In a conventional control apparatus for an internal combustion engine, a back-up control apparatus is well known, which is adapted to control an actuator, in place of a main control apparatus, for driving an equipment included in the internal combustion engine, when the main control apparatus becomes out of order. Such a conventional control apparatus is disclosed, for example, in JP-A- No. 59-196937 and JP-A- No. 60-339989. In the former control apparatus, in the case where a fault develops in a main throttle-control apparatus for controlling an actuator such as a servo motor for driving the throttle valve of an internal combustion engine mounted in a vehicle, the actuator is controlled by a back-up subcontrol apparatus thereby to secure the driving safety of the vehicle. The latter control apparatus, on the other hand, operates on principle that in the case where a fault develops in a control circuit for controlling an electromagnetic fuel injection valve in an internal combustion engine, pulse signals are produced from a pulse generator separated from the control circuit to control the electromagnetic fuel injection valve in place of the control circuit thereby to permit minimum drive. Assume, however, that the back-up control apparatuses disclosed by the prior arts develop a fault before the main control apparatus becomes out of order. The internal combustion engine would be controlled by the faulty back-up control apparatus, thus causing an unusual operation of the internal combustion engine. As a result, the vehicle would not be capable of performing relief drive, thereby making it impossible to secure the driving safety of the vehicle.